In moving image encoding, a method for enhancing encoding efficiency by removing correlation between pictures by using inter-picture prediction is used. For example, in moving image encoding schemes such as ISO (International Organization for Standardization), MPEG (Moving Picture Experts Group)-1/MPEG-2/MPEG-4/MPEG-4-AVC (Advanced Video Coding), and ITU-T (International Telecommunication Union Telecommunication Standardization Sector) H.264, prediction efficiency is enhanced by inter-picture prediction.
In FIG. 11, a typical configuration of a conventional moving image decoding apparatus performing inter-picture prediction block by block, the block obtained by dividing a screen, is shown.
A mechanism of decoding shown in FIG. 11 is as follows. An encoded bit string d1′ is decoded by an entropy decoder 1101, whereby motion vector information d2′ and coefficient information d3′ are obtained.
In a predictor 1102, a reference image d4′ that is an encoded image from a frame memory 1103 and the above-described motion vector d2′ are inputted thereto, and a predicted image d5′ is created.
On the other hand, in an inverse quantizer/inverter 1104, a difference image d6′ is obtained from the coefficient information d3′.
Then, in an adder 1105, the predicted image d5′ and the difference image d6′ are added, whereby a decoded image d7′ is obtained. The decoded image d7′ is stored in the frame memory 1103, and is used as a future reference image d4′.
In FIG. 12, a typical configuration of a conventional moving image encoding apparatus corresponding to the moving image decoding apparatus of FIG. 11 is shown.
A mechanism of encoding shown in FIG. 12 is as follows. First, in a predictor 1201, from motion vector information d2 and a reference image d4 read from a frame memory 1202, a predicted image d5 is created.
A difference between the predicted image d5 and an original image d0, the difference outputted from a differentiator 1203, is inputted to a converter/quantizer/inverse quantizer/inverter 1204, whereby a difference image d6 and coefficient information d3 are created.
In an adder 1205, the difference image d6 and the predicted image d5 are added, whereby a decoded image d7 is obtained. The decoded image d7 is stored in the frame memory 1202, and is used as a future reference image d4.
On the other hand, the coefficient information d3 is inputted to an entropy encoder 1206 along with the motion vector information d2, and is encoded into an encoded bit string d1.
As a related art, in JP-A-2007-184800, a technique of separating the entire screen of an original image into a high-frequency component and a low-frequency component, encoding each component, and decoding the images thereof by adding them at the time of decoding is disclosed. In this related art, enhancement of encoding efficiency by transmitting texture information (a high-frequency component and a low-frequency component) separately is disclosed.
Inter-picture prediction in image encoding makes it possible to enhance encoding efficiency by removing correlation between screens. However, with inter-picture prediction, correlation cannot always be removed completely. Specifically, in a part such as an edge part in which high-frequency components are concentrated, a prediction residual tends to remain depending on shooting conditions and conditions such as movement of a subject, characteristics of a camera, and the like, causing a reduction in encoding efficiency.
For example, the following discusses moving image encoding when phenomena shown in FIGS. 13A and 13B occur. In an image obtained by shooting a subject with an outline, the outline of the subject appears as an edge. However, a space frequency component of the edge changes with time depending on blurring due to movement of the subject, movement of a camera itself shooting the subject, and conditions such as focus.
In an example shown in FIG. 13A, as a result of movement of an original image that has remained stationary until now, a predicted image is a still image and includes a high-frequency component resulting from an edge. On the other hand, since the original image becomes an image including movement and an edge part becomes blurred, a high-frequency component resulting from an edge is reduced.
By contrast, in an example shown in FIG. 13B, as a result of an original image that has moved until now stopping moving, a predicted image is an image including movement and has a blurred edge part, and therefore there are few high-frequency components resulting from an edge. However, since the original image becomes a still image and an edge part becomes clear, a high-frequency component resulting from an edge is increased.
In each case, a frequency component varies between a predicted image and an original image, causing an increase in a prediction residual. This prediction residual cannot be adequately removed by a predicted image created by using a conventional low-pass filter or sinc filter having a fixed frequency characteristic.
Moreover, the technical trends demand that a prediction residual be reduced without changing a typical configuration of a moving image encoding apparatus, that is, a conventional encoding mechanism as little as possible.